Data from: Costs of immunity and their role in the range expansion of the house sparrow in Kenya

There are at least two reasons to study traits that mediate successful range expansions. First, dispersers will found new populations and thus impact the distribution and evolution of species. Second, organisms moving into new areas will influence the fate of resident communities, directly competing with or indirectly affecting residents by spreading non-native or spilling-back native parasites. The success of invaders in new areas is likely mediated by a counterbalancing of costly traits. In new areas where threats are comparatively rare, individuals that grow rapidly and breed prolifically should be at an advantage. High investment in defenses should thus be disfavored. In the present study, we compared the energetic, nutritional and collateral damage costs of an inflammatory response among Kenyan house sparrow (Passer domesticus) populations of different ages, asking whether costs were related to traits of individuals from three different capture sites. Kenya is among the world's most recent range expansions for this species, and we recently found that the expression of Toll-like receptors (TLRs), leukocyte receptors that instigate inflammatory responses when bound to microbial elements, was related to the range expansion across the country. Here, we found (contrary to our expectations) that energetic and nutritional costs of inflammation were higher, but damage costs were lower, in range-edge compared with core birds. Moreover, at the individual level, TLR-4 expression was negatively related to commodity costs (energy and a critical amino acid) of inflammation. Our data thus suggest that costs of inflammation, perhaps mediated by TLR expression, might mitigate successful range expansions.

探究介导物种成功分布范围扩张的性状，至少存在两方面核心动因。其一，扩散个体将建立新种群，进而影响物种的分布格局与演化进程；其二，迁入新区域的生物可通过直接竞争、传播外来寄生虫或回流本土寄生虫的间接途径，改变本土群落的存续命运。入侵物种在新区域的拓殖成功与否，大概率受携带代价的性状间的权衡调控。在威胁相对稀少的新环境中，生长快速、繁殖旺盛的个体将具备生存优势，因此对防御机制的高投入反而会处于劣势。本研究针对肯尼亚不同年龄家雀（Passer domesticus）种群的炎症反应所带来的能量成本、营养成本与附带损伤成本开展了比较分析，旨在探究这些成本是否与三个不同捕获位点的个体性状存在关联。家雀是全球范围内近期完成分布范围扩张的物种之一，肯尼亚正是其扩张的前沿区域之一；此前本团队的研究发现，当与微生物成分结合后可触发炎症反应的白细胞受体——toll样受体（Toll-like receptors, TLRs）的表达水平，与该物种在肯尼亚国内的分布范围扩张存在显著关联。本研究得到了与预期相悖的结果：相较于分布范围核心区域的家雀，分布在范围边缘的个体其炎症反应的能量与营养成本更高，而附带损伤成本更低。此外，在个体层面，TLR-4的表达水平与炎症反应的资源成本（能量与必需氨基酸）呈负相关。综上，本研究数据表明，炎症反应的成本（或许受toll样受体表达水平调控）可能会抑制物种的成功分布范围扩张。